The bidirectional exchanges between oocyte and contiguous cumulus cells (CCs) are important for oocyte competence acquisition, early embryonic development and CC expansion (Cha and Chian, 1998; Goud et al., 1998; Salustri et al., 1989). Oocyte maturation starts with the resumption of the first meiosis process, and is divided in nuclear and cytoplasmic maturation. During nuclear maturation, there is progression from prophase I characterized by germinal vesicle breakdown (GVBD) to metaphase II (MII) of the second meiosis (Cha and Chian, 1998; Wang and Sun, 2007). At the end of this process, the oocyte should be considered as mature and able to be fertilized.
However, the main problem, which hinders in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) success, is how to select oocytes competent for embryonic development and implantation. Gene expression profile of CCs has been suggested to predict embryo development and pregnancy outcome (Adriaenssens et al., 2010; Assou et al., 2008; Assou et al., 2010; Feuerstein et al., 2007; Hamel et al., 2008; Van Montfoort et al., 2008; Zhang et al., 2005). However, in the majority of these studies, they did not consider the possibility that CC gene expression profile might vary according to the stages of oocyte nuclear maturation and thus were focused mostly on a single specific phase of oocyte maturation, such as the MII stage (Feuerstein et al., 2007).
IVF is a powerful and widely used technique for the treatment of infertility. In this procedure, human eggs are retrieved and mixed with sperm in a culture dish to allow fertilization process. The embryos are then transferred to the uterus on day 2/3, when it has between 4 and 6-8 cells respectively or day 5 or 6 at blastocyst stage. This technique is used for women with, for example, damaged or absent Fallopian tubes, endometriosis, male factor infertility and unexplained infertility. However, the blastulation rate varies according the indications and male factor impact and this explain why implantation rate varies between 5% and 30%.
Under in vivo conception, the embryo reaches the uterus at a blastocyst stage of development. Accordingly, embryo coculture techniques, used successfully in animals, represent an effort to improve the culture media for embryos such that a greater proportion of embryos will reach the blastocyst stage for improving the implantation and pregnancy rates. In addition, if coculture results in a higher implantation rate, fewer embryos could be transferred at each cycle, resulting in a decreased incidence of multiple pregnancies. A variety of coculture techniques have been therefore investigated, involving the use of feeder cell layers derived from a range of tissues, including the use of human reproductive tissues (i.e., endometrium) and the coculture with cumulus cells (OMAR Farouk and Vlad, 2008; Quinn and Margalit, 1996 and EP0340934). However, no standardized method of growing an embryo to a blastocyst stage of development and no standardized method for coculture of embryo with cumulus cells for increasing the blastulation rate has emerged and the optimal system for preimplantation human embryo culture has not yet been determined.